Chromate-Free Coated Metal Sheet Gloss Control
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Solution Overview
Problem
Chromate-free coated metal sheets for exterior building materials face severe corrosion issues, particularly in flat portions, due to the use of silica particles as gloss adjusting agents, leading to stain rust and coating film blistering before the intended service life, which is not equivalent to the corrosion resistance of chromate-coated steel sheets.
Innovation Solution
A coated metal sheet with an overcoat coating film composed of fluorine resin and microporous particles as a gloss adjusting agent, where the mass ratio of fluorine resin to acrylic resin is 50:50 to 85:15, and the gloss adjusting agent content is between 0.01 to 15vol%, satisfying specific particle size distribution criteria to enhance corrosion resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If silica particles are used as gloss adjusting agent in chromate-free coated metal sheets, then designability with adjusted gloss is achieved, but corrosion resistance in flat portions deteriorates due to stain rust and coating film blistering
Solution Approach 1:
The invention changes the particle size parameters of the silica particles, specifically controlling the average particle diameter to 1 μm or less and the particle size distribution to satisfy specific relationships (D97.5 ≤ 3R and Ru ≤ 2R). This parameter optimization prevents the silica particles from protruding through the coating film while maintaining the desired gloss effect, thereby resolving the contradiction between designability and corrosion resistance.
Solution Approach 2:
The invention applies different quality requirements to different aspects of the silica particles: extremely fine particle size (average ≤1 μm) to prevent protrusion and corrosion, while maintaining sufficient particle presence (0.01-15 vol% content) to achieve the desired gloss effect. This localized optimization of particle properties allows simultaneous achievement of good gloss adjustment and corrosion resistance.
2Reliability
If chromate components are used in coated metal sheets, then corrosion resistance is improved, but environmental compatibility deteriorates due to restrictions on hexavalent chromium
Solution Approach 1:
The invention extracts and eliminates the harmful chromate components from the coating system while maintaining the essential function of corrosion resistance. By using ultra-fine silica particles (average diameter ≤1 μm) as the gloss adjusting agent with controlled particle size distribution, the invention achieves corrosion resistance without relying on chromate, thus resolving the contradiction between corrosion protection and environmental compatibility.
3Ease of manufacture
If larger silica particles are used as gloss adjusting agent, then gloss adjustment effect is enhanced, but particle exposure and cracking occur leading to severe corrosion
Solution Approach 1:
The invention dramatically reduces the particle size parameter to average diameter ≤1 μm, which is sufficiently small to remain embedded in the coating film without protruding or causing cracks. Despite the small size, the controlled particle size distribution (D97.5 ≤ 3R and Ru ≤ 2R) ensures adequate particle presence to achieve the desired gloss effect, thus preventing both particle exposure and maintaining designability.
Data Source
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AI summary
This coated metal sheet is for exterior covering, and has a metal sheet and a top coating layer disposed on the metal sheet. The top coating layer is configured from a fluororesin and contains 0.01-15 vol% of microporous particles as a gloss control agent, and the coated metal sheet satisfies the belowmentioned formulae. In the belowmentioned formulae, in the number-based particle size distribution of the gloss control agent, R is the number average particle size (µm), D97.5 is the 97.5% particle size (µm), Ru is the upper limit particle size (µm), and T is the top coating layer thickness (µm): D97.5/T ≤ 0.9; Ru ≤ 1.2T; R ≥ 1.0; and 3 ≤ T ≤ 40.